Semiconductor device and control method thereof

ABSTRACT

According to one embodiment, a DC-DC converter  1  includes a power supply unit  12  that includes an inductor L 1  and a switching unit and generates an output voltage Vout corresponding to a duty of a pulse signal P 1,  a PID controller  111  that outputs a control signal S corresponding to a difference between a divided voltage of Vout and a target voltage Vcnst, a PI controller  112  that outputs a control signal D corresponding to a difference between the control signal S and an average current flowing through the inductor L 1,  a PWM generation unit  113  that generates the pulse signal P 1  with a duty ratio corresponding to the control signal D, and in step-down mode, the PI controller  112  performs proportional control of the differential signal ei by using a product of the control signal D and a reference proportionality constant KP as a proportionality constant.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 14/991,399 filed Jan. 8,2016, and claims priority from Japanese patent application No.2015-033926, filed on Feb. 24, 2015, the disclosures of which areincorporated by reference herein in their entirety.

BACKGROUND

The present invention relates to a semiconductor device and a controlmethod thereof and, for example, relates to a semiconductor device and acontrol method thereof suitable for generating a stable output voltage.

The step-up/step-down power supply disclosed in Linear TechnologyCorporation [online] [Searched on Jan. 15, 2015] Internet<URL:http://cds.linear.com/docs/en/datasheet/3129fb.pdf> and TexasInstruments Incorporated [online] [Searched on Jan. 15, 2015] Internet<URL:http://www.ti.com/lit/ds/symlink/tps63060.pdf> employs averagecurrent mode control, which is one way of output voltage control. Thestep-up/step-down power supply that employs this control method includesa voltage loop for feedback control of an output voltage and a currentloop for feedback control of an average current flowing through aninductor. The voltage loop is used mainly to suppress variation of anoutput voltage due to a load connected to an output terminal. Thecurrent loop is used mainly to suppress variation of an output voltagedue to variation of an input voltage.

SUMMARY

In the configuration disclosed in the above-described related art, acurrent flowing through the inductor is proportional to the inputvoltage in step-down mode, which is, when input voltage≧output voltageis satisfied. Accordingly, the bandwidth of the current loop forfeedback control of a current flowing through the inductor is alsoproportional to the input voltage. Therefore, while the bandwidth of thecurrent loop is large when the input voltage is high, the bandwidth ofthe current loop becomes smaller as the input voltage becomes lower.Thus, in this configuration, it is difficult to achieve the largebandwidth of the current loop all over the range of the input voltage.Note that, when the bandwidth of the current loop is small, variation ofthe output voltage due to variation of the input voltage increases,which causes the output voltage to be unstable.

The other problems and novel features of the present invention willbecome apparent from the description of the specification and theaccompanying drawings.

According to one embodiment, a semiconductor device includes a powersupply unit that includes an inductor and a switching unit turning onand off under control by a pulse signal to control a current flowingthrough the inductor, and generates an output voltage by changing aninput voltage by an amount of voltage corresponding to a duty ratio ofthe pulse signal, a first control unit that performs PI control of afirst differential signal being a difference between a comparisonvoltage corresponding to the output voltage and a target voltage andoutputs a first control signal, a second control unit that performs PIcontrol of a second differential signal being a difference between thefirst control signal and a current signal indicating an average value ofthe current flowing through the inductor and outputs a second controlsignal, and a PWM generation unit that generates the pulse signal with aduty ratio corresponding to the second control signal, wherein, instep-down mode, the second control unit performs proportional control ofthe second differential signal by using a multiplication result ofmultiplying the second control signal by a reference proportionalityconstant as a proportionality constant.

According to one embodiment, a control method of a semiconductor deviceis a control method of a semiconductor device that controls a currentflowing through an inductor by a pulse signal and thereby generates anoutput voltage by changing an input voltage by an amount of voltagecorresponding to a duty ratio of the pulse signal, and the methodincludes performing PI control of a first differential signal being adifference between a comparison voltage corresponding to the outputvoltage and a target voltage and outputting a first control signal, instep-down mode, performing proportional control of a second differentialsignal being a difference between the first control signal and a currentsignal indicating an average value of the current flowing through theinductor by using a multiplication result of multiplying a secondcontrol signal by a reference proportionality constant as aproportionality constant, and performing integral control of the seconddifferential signal and outputting the second control signal, andgenerating the pulse signal with a duty ratio corresponding to thesecond control signal.

According to the above-described embodiment, it is possible to provide asemiconductor device capable of generating a stable output voltagewithout depending on the level of an input voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, advantages and features will be moreapparent from the following description of certain embodiments taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a view showing a configuration of a DC-DC converter accordingto a first embodiment.

FIG. 2 is a view showing a configuration of a DC-DC converter accordingto a second embodiment.

FIG. 3 is a view showing a configuration of a DC-DC converter accordingto a third embodiment.

FIG. 4 is a view showing a part of a configuration of a DC-DC converteraccording to a fourth embodiment.

DETAILED DESCRIPTION

Embodiments of the present invention are described hereinafter withreference to the drawings. It should be noted that the drawings aregiven in simplified form by way of illustration only, and thus are notto be considered as limiting the present invention. The same elementsare denoted by the same reference symbols, and the redundant explanationis omitted.

In the following embodiments, the description will be divided into aplurality of sections or embodiments when necessary for the sake ofconvenience. However, unless explicitly specified otherwise, thosesections or embodiments are by no means unrelated to each other, but arein such a relation that one represents a modification, a detailed orsupplementary description, etc. of part or whole of the other. Further,in the following embodiments, when a reference is made to the numberetc, (including the number, numeric value, quantity, range, etc.) ofelements, except in such cases where it is explicitly specifiedotherwise or the number is obviously limited to a specific number inprinciple, the number is not limited to the specific number but may begreater or less than the specific number.

It is needless to mention that, in the following embodiments, theirconstituent elements (including operation steps) are not necessarilyessential, except in such cases where it is explicitly specifiedotherwise or they are obviously considered to be essential in principle.Likewise, in the following embodiments, when a reference is made to theshape, relative position, etc. of a constituent element or the like,this includes those shapes etc. substantially resembling or similar tothat shape etc., except in such cases where it is explicitly specifiedotherwise or it is obviously considered otherwise in principle. The sameapplies to the number etc, (including the number, numeric value,quantity, range, etc.) mentioned above.

First Embodiment

FIG. 1 is a view showing a configuration of a DC-DC converter(semiconductor device) 1 according to a first embodiment. The DC-DCconverter 1 is mounted on a vehicle and used to supply a stable voltageto an on-vehicle analog product, for example.

As shown in FIG. 1, the DC-DC converter 1 includes a power supply unit12 and a control unit 11.

(Power Supply Unit 12)

The power supply unit 12 is a part that steps up or steps down an inputvoltage Vin by the amount of voltage corresponding to a duty ratio ofpulse signals P1 and P2 that are supplied from the control unit 11 andthereby generates an output voltage Vout.

Specifically, the power supply unit 12 includes transistors Tr1 to Tr4that constitute a switching unit, an inductor L1, a capacitor C1, andresistors R1 and R2. In this embodiment, the case where all of thetransistors Tr1 to Tr4 are N-channel MOS transistors is described as anexample.

In the transistor Tr1, the drain is connected to the input terminal IN,the source is connected to one end (node N1) of the inductor L1, and thepulse signal P1 from the control unit 11 is supplied to the gate. In thetransistor Tr2, the drain is connected to the node N1, the source isconnected to a ground voltage terminal GND, and an inverted signal ofthe pulse signal P1 from the control unit 11 is supplied to the gate.Note that the input voltage Vin is supplied to the input terminal N1from the outside. A ground voltage GND is supplied to the ground voltageterminal GND.

In the transistor Tr3, the source is connected to the output terminalOUT, the drain is connected to the other end (node N2) of the inductorL1, and an inverted signal of the pulse signal P2 from the control unit11 is supplied to the gate. In the transistor Tr4, the source isconnected to the ground voltage terminal GND, the drain is connected thenode N2, and the pulse signal P2 from the control unit 11 is supplied tothe gate. Note that the output voltage Vout is output from outputterminal OUT to the outside (load).

The capacitor C1 is placed between the output terminal OUT and theground voltage terminal GND. The resistors R1 and R2 are placed inseries between the output terminal OUT and the ground voltage terminalGND. Note that a voltage (comparison voltage) Vfb of a node N3 betweenthe resistors R1 and R2 is generated by dividing the output voltage Voutby the resistors R1 and R2 and is fed back to the control unit 11.Further, a current Ifb flowing through the inductor L1 is also fed backfrom one end (node N1) of the inductor L1 to the control unit 11.

(Basic Operation of Power Supply Unit 12)

The basic operation of the power supply unit 12 is briefly describedhereinbelow.

In step-down mode, the transistor Tr3 is fixed to ON, and the transistorTr4 is fixed to OFF. Then, the transistor Tr1 turns ON and thetransistor Tr2 turns OFF first, and thereby a current flows from theinput terminal IN to the output terminal OUT through the transistor Tr1and the inductor L1. Current energy is stored in the inductor L1 at thistime. After that, the transistor Tr1 turns OFF and the transistor Tr2turns ON, and thereby the current that has been flowing from the inputterminal IN to the inductor L1 through the transistor Tr1 is shut off.The inductor L1 then releases the stored current energy to the outputterminal OUT in order to maintain the current value of the current thathas been flowing just before. A current thereby flows from the groundvoltage terminal GND to the output terminal OUT through the transistorTr2. By repeating such an operation, the power supply unit 12 generatesthe output voltage Vout by stepping down the input voltage Vin by thelevel corresponding to the duty ratio of the pulse signal P1.

On the other hand, in step-up mode, the transistor Tr1 is fixed to ON,and the transistor Tr2 is fixed to OFF. Then, the transistor Tr4 turnsON and the transistor Tr3 turns OFF first, and thereby a current flowsfrom the input terminal IN to the ground voltage terminal GND throughthe inductor L1 and the transistor Tr4. Current energy is stored in theinductor L1 at this time. After that, the transistor Tr4 turns OFF andthe transistor Tr3 turns ON, and thereby the current that has beenflowing from the inductor L1 to the ground voltage terminal GND throughthe transistor Tr4 is shut off. The inductor L1 then flows the storedcurrent energy to the output terminal OUT in order to maintain thecurrent value of the current that has been flowing just before. Byrepeating such an operation, the power supply unit 12 generates theoutput voltage Vout by stepping up the input voltage Vin by the levelcorresponding to the duty ratio of the pulse signal P2.

In step-up/step-down mode, the operation in step-up mode and theoperation in step-down mode are performed in combination.

(Control Unit 11)

The control unit 11 is a part that outputs the pulse signals P1 and P2for controlling the step-up or step-down level of the power supply unit12.

Specifically, the control unit 11 includes a PID controller (firstcontrol unit) 111, a PI controller (second control unit) 112, a PWMgeneration unit 113, a current detection unit 114, a filter 115, astep-up/step-down determination unit (determination circuit) 116,subtracters 117 and 118, a storage unit 119, a multiplier 120, aselector (first selection circuit) 121, and inverters INV1 and INV2.

The subtracter 117 outputs a difference between a target voltage Vcnstthat can be set arbitrarily and a voltage Vfb that is fed back from thepower supply unit 12 as a differential signal (first differentialsignal) e.

The PID controller 111 is a circuit that performs feedback control ofthe output voltage Vout, and it performs PID control (proportionalcontrol, integral control and derivative control) of the differentialsignal e that is output from the subtracter 117 and outputs a result asa control signal (first control signal) S.

Note that, in the PID controller 111, proportional control, integralcontrol and derivative control on the differential signal e arerespectively performed based on the following expressions (1), (2) and(3), where KP indicates a reference proportionality constant, KIindicates an integral constant, KD indicates derivative constant, and tindicates time.

Proportional control: KP×e(t)  (1)

Integral control: KI×∫e(t)dt  (2)

Derivative control: KD×de(t)dt  (3)

Then, the PID controller 111 adds up results of performing proportionalcontrol, integral control and derivative control on the differentialsignal e and outputs a result of the addition as the control signal S.

The current detection unit 114 detects an average current Ifb that flowsthrough the inductor L1 from one end (node N1) of the inductor L1. Thecurrent Ifb is rectified by the filter 115.

The subtracter 118 outputs a difference between the control signal Sthat is output from the PID controller 111 and the current Ifb that isfed back from the power supply unit 12 as a differential signal (seconddifferential signal) ei.

The PI controller 112 is a circuit that performs feedback control of theaverage current Ifb that flows through the inductor L1, and it performsPI control (proportional control and integral control) of thedifferential signal ei that is output from the subtracter 118 andoutputs a result as a control signal (second control signal) D.

In step-down mode, which is, when input voltage Vin output voltage Voutis satisfied, the average current Ifb that flows through the inductor L1is proportional to the input voltage Vin. Thus, if no measures aretaken, the bandwidth of the current loop for feedback control of thecurrent flowing through the inductor L1 is also proportional to theinput voltage Vin, which makes it difficult to increase the bandwidth ofthe current loop. Thus, the inventor of the present invention hasfocused attention on the fact that the control signal D is inverselyproportional to the input voltage Vin in step-down mode and employed aconfiguration of performing proportional control of the differentialsignal ei by using a proportionality constant obtained by multiplyingthe reference proportionality constant KP by the control signal D. Thus,in the DC-DC converter 1, the dependence of the bandwidth of the currentloop on the input voltage is canceled out in step-down mode, and it isthereby possible to generate the stable output voltage Vout withoutdepending on the level of the input voltage Vin. Note that, in step-upmode, the bandwidth of the current loop does not have the dependence onthe input voltage.

To be specific, the step-up/step-down determination unit 116 determineswhether the power supply unit 12 is in step-up mode or in step-down modebased on the control signal D that is output from the PI controller 112.The multiplier 120 multiplies the reference proportionality constant KPstored in the storage unit 119 by the control signal D and outputs aresult KP×D. The selector 121 selects and outputs any one of thereference proportionality constant KP and the multiplication result KP×Dbased on a determination result of the step-up/step-down determinationunit 116. For example, when the step-up/step-down determination unit 116determines that it is in step-up mode (or in step-up/step-down mode),the selector 121 selects and outputs the reference proportionalityconstant KP. On the other hand, when the step-up/step-down determinationunit 116 determines that it is in step-down mode, the selector 121selects and outputs the multiplication result KP×D.

The output result of the selector 121 is used as a proportionalityconstant for proportional control by the PI controller 112.Specifically, the PI controller 112 performs proportional control of thedifferential signal ei by using the reference proportionality constantKP as a proportionality constant in step-up mode, and performsproportional control of the differential signal ei by using themultiplication result KP×D as a proportionality constant in step-downmode.

Note that, in the PI controller 112, proportional control and integralcontrol on the differential signal ei are performed based on thefollowing expressions (4) and (5), respectively.

Proportional control: (in step-up mode): KP×ei(t) (in step-down mode):KP×D×ei(t)  (4)

Integral control: KI×∫ei(t)dt  (5)

Then, the PI controller 112 adds up results of performing proportionalcontrol and integral control on the differential signal ei and outputs aresult of the addition as the control signal D.

The PWM generation unit 113 generates pulse signals P1 and P2 with aduty ratio according to the control signal D. The pulse signal P1 issupplied to the gate of the transistor Tr1 in the power supply unit 12,and is also inverted by the inverter INV1 and supplied to the gate ofthe transistor Tr2 in the power supply unit 12. The pulse signal P2 issupplied to the gate of the transistor Tr4 in the power supply unit 12,and is also inverted by the inverter INV2 and supplied to the gate ofthe transistor Tr3 in the power supply unit 12.

As described above, in the DC-DC converter 1 according to thisembodiment, in step-down mode, proportional control is performed on thedifferential signal ei (current loop) using a proportionality constantmultiplied by the control signal D that is inversely proportional to theinput voltage Vin. The DC-DC converter 1 can thereby cancel out thedependence of the bandwidth of the current loop on the input voltage instep-down mode, and it is thereby possible to increase the bandwidth ofthe current loop all over the range of the input voltage. Further, withan increase in the bandwidth of the current loop, it is also possible toincrease the bandwidth of the voltage loop for feedback control of theoutput voltage Vout. Consequently, the DC-DC converter 1 can generatethe stable output voltage Vout without depending on the level of theinput voltage Vin. In other words, it is possible to improve the linetransient characteristic and the load transient characteristic.

Although the case where the DC-DC converter 1 is a step-up/step-downtype is described as an example in this embodiment, it is not limitedthereto, and any type of converter may be used as long as it at leasthas a function of stepping down a voltage.

Further, the PID controller 111 may be replaced with a PI controllerthat performs proportional control and integral control only.

Further, the PI controller 112 may be replaced with a PID controllerthat performs derivative control in addition to proportional control andintegral control.

Further, the PI controller 112 may perform not only proportional controlusing a proportionality constant multiplied by the control signal D butalso integral control using an integral constant multiplied by thecontrol signal D in step-down mode. Furthermore, in the case where thePI controller 112 is replaced with a PID controller, it may performderivative control using a derivative constant multiplied by the controlsignal D in step-down mode.

Second Embodiment

FIG. 2 is a view showing a configuration of a DC-DC converter 2according to a second embodiment.

As shown in FIG. 2, the DC-DC converter 2 is a digital-control DC-DCconverter, and it includes a control unit 21 in place of the controlunit 11.

The control unit 21 includes a PID controller 211, a PI controller 212,a PWM generation unit 213, a current detection unit 214, a filter 215, astep-up/step-down determination unit 216, subtracters 217 and 218, astorage unit 219, a multiplier 220, a selector 221, inverters INV1 andINV2, a flip-flop 222, and A-D converters 223 and 224.

Note that the PID controller 211, the PI controller 212, the PWMgeneration unit 213, the current detection unit 214, the filter 215, thestep-up/step-down determination unit 216, the subtracters 217 and 218,the storage unit 219, the multiplier 220, the selector 221 and theinverters INV1 and INV2 in the control unit 21 respectively correspondto the PID controller 111, the PI controller 112, the PWM generationunit 113, the current detection unit 114, the filter 115, thestep-up/step-down determination unit 116, the subtracters 117 and 118,the storage unit 119, the multiplier 120, the selector 121 and theinverters INV1 and INV2 in the control unit 11.

FIG. 2 shows a detailed configuration of the PI controller 212.Specifically, the PI controller 212 includes multipliers 225 and 226,adders 227 and 230, a storage unit 228, and a flip-flop 229.

The A-D converter 223 converts the differential signal e that is outputfrom the subtracter 217 from analog to digital and outputs a result. ThePID controller 211 performs PID control of the digital differentialsignal e and outputs a result as a digital control signal S. The A-Dconverter 224 converts the average current Ifb that flows through theinductor L1 that is output from the filter 215 from analog to digitaland outputs a result. The subtracter 218 outputs a difference betweenthe digital control signal S and the digital current signal representingthe average current Ifb as a digital differential signal ei.

The flip-flop 222 latches the control signal (digital code) D insynchronization with the rising edge of a clock signal and outputs it asa control signal Dz. Note that the flip-flop 222 may have any structureas long as it delays the control signal D by one clock cycle and thenoutputs it.

The multiplier 220 multiplies the reference proportionality constant KPstored in the storage unit 219 by the control signal (digital code) Dzand outputs a result KP×Dz. The selector 221 selects and outputs any oneof the reference proportionality constant KP and the multiplicationresult KP×Dz based on a determination result of the step-up/step-downdetermination unit 216.

The step-up/step-down determination unit 216 determines whether thepower supply unit 12 is in step-up mode, in step-down mode, or instep-up/step-down mode based on the control signal Dz. For example, thestep-up/step-down determination unit 216 outputs a determination resultwith a value 0 indicating step-down mode when the control signal Dz,which is a 6-bit digital code, indicates 0 to 31 in decimal notation,and outputs a determination result with a value 1 indicating step-upmode or step-up/step-down mode when the control signal Dz indicates 32to 63 in decimal notation. Thus, when the step-up/step-downdetermination unit 216 outputs a determination result with a value 0,the selector 221 selects and outputs the multiplication result KP×Dz,and when the step-up/step-down determination unit 216 outputs adetermination result with a value 1, the selector 221 selects andoutputs the reference proportionality constant KP.

In the PI controller 212, the multiplier 225 forms a circuit thatperforms proportional control of the differential signal ei, and itoutputs a result of multiplying the differential signal ei by the outputof the selector 221. Specifically, the multiplier 225 multiplies thedifferential signal ei by the reference proportionality constant KP andoutputs a result in step-up mode (or in step-up/step-down mode), andmultiplies the differential signal ei by the multiplication result KP×Dzand outputs a result in step-down mode.

Further, in the PI controller 212, the multiplier 226, the storage unit228, the adder 227 and the flip-flop 229 form a circuit that performsintegral control of the differential signal ei. The multiplier 226outputs a result of multiplying the differential signal ei by theintegral constant KI stored in the storage unit 228. The adder 227 addsup a result of multiplication by the multiplier 226 and data latched bythe flip-flop 229 and outputs it. The flip-flop 229 latches and outputsthe output of the adder 227. Thus, a result of multiplication of thedifferential signal ei by the integral constant KI is integrated by theadder 227 and the flip-flop 229.

Then, in the PI controller 212, the adder 230 adds up the output of themultiplier 225 (a result of proportional control) and the output of theflip-flop 229 (a result of integral control) and outputs it as thecontrol signal (digital code) D.

The other configuration and operation of the DC-DC converter 2 arebasically the same as those of the DC-DC converter 1 and thus notredundantly described.

As described above, in the DC-DC converter 2 according to thisembodiment, in step-down mode, proportional control is performed on thedifferential signal ei (current loop) using a proportionality constantmultiplied by the control signal Dz that is inversely proportional tothe input voltage Vin. The DC-DC converter 2 can thereby cancel out thedependence of the bandwidth of the current loop on the input voltage instep-down mode, and it is thereby possible to increase the bandwidth ofthe current loop all over the range of the input voltage. Further, withan increase in the bandwidth of the current loop, it is also possible toincrease the bandwidth of the voltage loop. Consequently, the DC-DCconverter 2 can generate the stable output voltage Vout withoutdepending on the level of the input voltage Vin. In other words, it ispossible to improve the line transient characteristic and the loadtransient characteristic.

Although the case where the DC-DC converter 2 is a step-up/step-downtype is described as an example in this embodiment, it is not limitedthereto, and any type of converter may be used as long as it at leasthas a function of stepping down a voltage.

Further, the PID controller 211 may be replaced with a PI controllerthat performs proportional control and integral control only.

Further, the PI controller 212 may be replaced with a PID controllerthat performs derivative control in addition to proportional control andintegral control.

Further, the PI controller 212 may perform not only proportional controlusing a proportionality constant multiplied by the control signal Dz butalso integral control using an integral constant multiplied by thecontrol signal Dz in step-down mode. Furthermore, in the case where thePI controller 212 is replaced with a PID controller, it may performderivative control using a derivative constant multiplied by the controlsignal Dz in step-down mode.

Third Embodiment

FIG. 3 is a view showing a configuration of a DC-DC converter 3according to a third embodiment.

As shown in FIG. 3, the DC-DC converter 3 is a digital-control DC-DCconverter, and it includes a control unit 31 in place of the controlunit 21.

The control unit 31 includes a PID controller 311 in place of the PIDcontroller 211, and further includes a gain unit 331 and a selector(second selection circuit) 332.

FIG. 3 shows a detailed configuration of the PID controller 311.Specifically, the PID controller 311 includes a multiplier 333, anintegrator 334, a differentiator 335, and an adder 336.

The gain unit 331 amplifies (multiplies) the reference proportionalityconstant KP that is stored in the storage unit 219 by a specified gainand outputs a result. The selector 332 selects and outputs any one ofthe reference proportionality constant KP and the output of the gainunit 331 (a proportionality constant obtained by amplifying thereference proportionality constant KP) based on a determination resultof the step-up/step-down determination unit 216.

In the PID controller 311, the multiplier 333 forms a circuit thatperforms proportional control of the differential signal e, and itoutputs a result of multiplying the differential signal e by the outputof the selector 332. Specifically, the multiplier 333 multiplies thedifferential signal e by the reference proportionality constant KP andoutputs a result in step-up mode (or in step-up/step-down mode), andmultiplies the differential signal e by the proportionality constantobtained by amplifying the reference proportionality constant KP andoutputs a result in step-down mode.

Further, in the PID controller 311, the integrator 334 performs integralcontrol of the differential signal e, and the differentiator 335performs derivative control of the differential signal e. Then, theadder 336 adds up the output of the multiplier 333 (a result ofproportional control), the output of the integrator 334 (a result ofintegral control) and the output of the differentiator 335 (a result ofderivative control), and outputs a result of the addition as the controlsignal S.

The other configuration and operation of the DC-DC converter 3 arebasically the same as those of the DC-DC converter 2 and thus notredundantly described.

In step-up mode, there is a possibility that RHPZ (Right Half PlaneZero) occurs in the power supply unit 12, and in this case, thebandwidth of the voltage loop is limited by the RHPZ frequency. As aresult, the bandwidth of the voltage loop is limited also in step-downmode. In view of this, in the DC-DC converter 3 according to thisembodiment, the PID controller 311 performs proportional control byusing a proportionality constant that is amplified by a specified gainin step-down mode. The DC-DC converter 3 can thereby prevent reductionof the bandwidth of the voltage loop in step-down mode.

As described above, the DC-DC converter 3 according to this embodimenthas substantially the same advantageous effects as the DC-DC converter 2according to the second embodiment. Additionally, in the DC-DC converter3, the PID controller 311 performs proportional control by using aproportionality constant that is amplified by a specified gain instep-down mode. The DC-DC converter 3 can thereby prevent reduction ofthe bandwidth of the voltage loop in step-down mode.

Although the case where the DC-DC converter 3 is a step-up/step-downtype is described as an example in this embodiment, it is not limitedthereto, and any type of converter may be used as long as it at leasthas a function of stepping down a voltage.

Further, the PID controller 311 may be replaced with a PI controllerthat performs proportional control and integral control only.

Further, the PID controller 311 may perform not only proportionalcontrol using a proportionality constant amplified by a specified gainbut also integral control and derivative control using an integralconstant and a derivative constant amplified by a specified gain as wellin step-down mode.

Fourth Embodiment

FIG. 4 is a view showing a part of a configuration of a DC-DC converter4 according to a fourth embodiment.

The DC-DC converter 4 is an analog-control DC-DC converter.

Note that FIG. 4 shows only the part corresponding to the PI controller112 in FIG. 1 and a feedback path of the control signal D. Specifically,FIG. 4 shows a PI controller 412 and a buffer 416 and a limiter 417 thatare placed on the feedback path of the control signal D.

The PI controller 412 includes an amplifier 413, a variable resistor414, and a capacitor 415. The amplifier 413 amplifiers a potentialdifference between the control signal S that is output from the PIDcontroller 211 and the current signal indicating the average current Ifbthat flows through the inductor L1 and outputs it as the control signalD. The variable resistor 414 and the capacitor 415 are placed in seriesbetween the output of the amplifier 413 and the ground voltage terminalGND. A resistance value of the variable resistor 414 is controlled bythe control signal D that is fed back through the buffer 416 and thelimiter 417.

A proportionality constant that is used for proportional control by thePI controller 412 is determined by the resistance value of the variableresistor 414. Accordingly, by controlling the resistance value of thevariable resistor 414 by the control signal D that is inverselyproportional to the input voltage Vin, the DC-DC converter 4 can cancelout the dependence of the bandwidth of the current loop on the inputvoltage in step-down mode. It is thereby possible to increase thebandwidth of the current loop all over the range of the input voltage.Further, with an increase in the bandwidth of the current loop, it isalso possible to increase the bandwidth of the voltage loop.Consequently, the DC-DC converter 4 can generate the stable outputvoltage Vout without depending on the level of the input voltage Vin. Inother words, it is possible to improve the line transient characteristicand the load transient characteristic.

Although embodiments of the present invention are described specificallyin the foregoing, the present invention is not restricted to theabove-described embodiments, and various changes and modifications maybe made without departing from the scope of the invention.

For example, in the semiconductor device according to the aboveembodiment, the conductivity type (P type or N type) of a semiconductorsubstrate, a semiconductor layer, a diffusion layer (diffusion region)and the like may be inverted. Accordingly, when one conductivity type ofN type and P type is a first conductivity type and the otherconductivity type thereof is a second conductivity type, the firstconductivity type may be P type and the second conductivity type may beN type, or the first conductivity type may be N type and the secondconductivity type may be P type on the contrary.

The first to fourth embodiments can be combined as desirable by one ofordinary skill in the art.

While the invention has been described in terms of several embodiments,those skilled in the art will recognize that the invention can bepracticed with various modifications within the spirit and scope of theappended claims and the invention is not limited to the examplesdescribed above.

Further, the scope of the claims is not limited by the embodimentsdescribed above.

Furthermore, it is noted that, Applicant's intent is to encompassequivalents of all claim elements, even if amended later duringprosecution.

What is claimed is:
 1. A semiconductor device comprising: a power supplyunit that includes an inductor and a switching unit turning on and offunder control by a pulse signal to control a current flowing through theinductor, and generates an output voltage by changing an input voltageby an amount of voltage corresponding to a duty ratio of the pulsesignal; a first control unit that performs PI control of a firstdifferential signal being a difference between a comparison voltagecorresponding to the output voltage and a target voltage and outputs afirst control signal; a second control unit that performs PI control ofa second differential signal being a difference between the firstcontrol signal and a current signal indicating an average value of thecurrent flowing through the inductor and outputs a second controlsignal; and a PWM generation unit that generates the pulse signal with aduty ratio corresponding to the second control signal, wherein instep-down mode, the second control unit performs proportional control ofthe second differential signal by using a multiplication result ofmultiplying the second control signal by a reference proportionalityconstant as a proportionality constant.
 2. The semiconductor deviceaccording to claim 1, wherein, in step-up mode, the second control unitperforms proportional control of the second differential signal by usingthe reference proportionality constant as a proportionality constant. 3.The semiconductor device according to claim 2, further comprising: adetermination circuit that determines whether it is in step-down mode orin step-up mode; and a first selection circuit that selects and outputsany one of the multiplication result and the reference proportionalityconstant based on a determination result of the determination circuit,wherein the second control unit performs proportional control of thesecond differential signal by using an output result of the firstselection circuit as a proportionality constant.
 4. The semiconductordevice according to claim 3, wherein the second control unit is a PIcontroller that performs proportional control of the second differentialsignal by using an output result of the first selection circuit as aproportionality constant and further performs integral control of thesecond differential signal and outputs the second control signal.
 5. Thesemiconductor device according to claim 1, wherein the second controlunit is a PI controller that performs, in step-down mode, proportionalcontrol of the second differential signal by using the multiplicationresult as a proportionality constant and further performs integralcontrol of the second differential signal and outputs the second controlsignal.
 6. The semiconductor device according to claim 3, furthercomprising: a second selection circuit that selects and outputs any oneof the reference proportionality constant and a constant obtained byamplifying the reference proportionality constant by a specified gainbased on a determination result of the determination circuit, whereinthe first control unit performs proportional control of the firstdifferential signal by using an output result of the second selectioncircuit as a proportionality constant.
 7. The semiconductor deviceaccording to claim 1, wherein the second control unit includes: anamplifier that amplifies a potential difference between the secondcontrol signal being an analog signal and the current signal being ananalog signal and outputs the second control signal; a capacitor that isplaced between an output terminal of the amplifier and a ground voltageterminal; and a variable resistor that is placed between the outputterminal of the amplifier and the capacitor and has a resistance valuevarying according to the second control signal.
 8. A control method of asemiconductor device that controls a current flowing through an inductorby a pulse signal and thereby generates an output voltage by changing aninput voltage by an amount of voltage corresponding to a duty ratio ofthe pulse signal, the method comprising: performing PI control of afirst differential signal being a difference between a comparisonvoltage corresponding to the output voltage and a target voltage andoutputting a first control signal; in step-down mode, performingproportional control of a second differential signal being a differencebetween the first control signal and a current signal indicating anaverage value of the current flowing through the inductor by using amultiplication result of multiplying a second control signal by areference proportionality constant as a proportionality constant, andperforming integral control of the second differential signal andoutputting the second control signal; and generating the pulse signalwith a duty ratio corresponding to the second control signal.
 9. Thecontrol method of a semiconductor device according to claim 8,comprising: in step-up mode, performing proportional control of thesecond differential signal by using the reference proportionalityconstant as a proportionality constant and further performing integralcontrol of the second differential signal and outputting the secondcontrol signal.
 10. The control method of a semiconductor deviceaccording to claim 9, comprising: determining whether it is in step-downmode or in step-up mode; selecting any one of the multiplication resultand the reference proportionality constant based on a result of thedetermination; and performing proportional control of the seconddifferential signal by using the selected one as a proportionalityconstant.